Method of coating fluorescent layer of electron discharge tube



Oct. 11, 1966 J, CGEE 3,278,326

METHOD OF COATING FLUORESCENT LAYER OF ELECTRON DISCHARGE TUBE OriginalFiled May 2, 1962 BVOLS/OA United States Patent 3,278,326 METHOD OFCOATING FLUORESCENT LAYER OF ELECTRON DISCHARGE TUBE James Dwyer McGee,London, England, assignor to National Research Development Corporation,London, England Original application May 2, 1962, Ser. No. 191,844. andthis application July 7, 1965, Ser. No.

Claims priority, application Great Britain, May 4, 1961,

36/61 Claims. c1.117-33.s

This application is a division of my copendin-g application Serial No.191,844, filed May 2, 1962 now abandoned.

This invention relates to electron discharge tubes employing a phosphorscreen which is backed by a thin aluminum film.

It is well-known technique in such electron discharge tubes to provide afluorescent screen by applying a thin layer of crystalline phosphorpowder to the output endface of the tube, through which end-face thephosphor screen is viewed. A thin, electron-transparent but opticallynearly opaque aluminum film is then applied to the phosphor screensurface remote from the tube end-face.

Such an aluminum backing film is intended to have three functions.

First, when the phosphor screen is excited by incident electrons, somelight is emitted from the screen in the direction away from theobserver. An aluminum backing layer reflects such light back to thedirection in which the screen is viewed. Provided that the aluminumlayer is in intimate contact with the phosphor layer, little lightdispersion occurs and image definition is but little impaired.

Second, as the aluminum layer is nearly opaque, light is substantiallyprevented from leaving the back surface of the phosphor screen.Otherwise, :such light, if presented in a tube using a photocathode,would result in a spurious, background illumination of the photocathodeand unwanted consequent photo-electron emission. In a cathode ray tube,such light would be scattered back to the phosphor screen and therebyreduce image contrast.

Third, the aluminum film serves as an electrically conducting electrodeto maintain the screen potential uniform over its area. In the absenceof an aluminum backing film, the screen potential has to be maintainedby secondary emission from the screen itself. This process is unreliablebecause, at not very high potentials, the secondary emission coeflicientmay already have fallen to less than unity.

The conventional aluminum film backing, as provided by the conventionalmethod, is subject to disadvantages which are discussed in detail below.

The object of the present invention is to provide an improved method ofapplying an aluminum backing film to a phosphor screen and an improvedelectron discharge tube having an aluminum backing film thus applied.

Accordingly, one form of the invention provides an electron dischargetube having a fluorescent layer disposed on a transparent carrier andfirst and second electron-permeable backing layers disposed insuccession on the face of the fluorescent layer remote from the carrier,the first backing layer being a discontinuous, high lightreflecting,metallic layer in intimate local contact with said fluorescent layer andthe second backing layer being, continuous and electrically conductive.

Another form of the invention provides a method of providinglight-reflecting and electrically-conductive backing layers for thefluorescent layer of an electron tube as described above, comprisingevaporating a first metallic backing layer onto discontinuous areas ofsaid fluorescent layer and subsequently disposing a second, continuous,metallic, backing layer over the first backing layer.

In order that the invention may readily be carried into effect, theproblem with which the present invention deals and a preferredembodiment of the invention, by way of example, will now be describedwith reference to the accompanying drawings, of which:

FIG. 1 is a diagram showing, not to scale, a small section of theend-face of a known electron discharge tube, and

FIG. 2 is a similar diagram showing a variant provided by the presentinvention.

FIG. 1 shows a small section of the glass end-face 1 of the envelope ofan electron discharge tube. On the inner surface A of the end-face 1 isdeposited a crystalline phosphor screen 2. On the inner face of thephosphor screen 2, is provided a thin aluminum film 3.

I By way of example, the screen 2 is viewed by a photographic emulsionlayer 4, arranged in close contact with the outside surface B of thetube face 1.

The path of an incident electron is shown by the full line 5. This lineis produced through the tube face 1 and through the emulsion layer 4 bya dotted line 5. A divergent path, from the phosphor screen 2 to theemulsion layer 4, of a ray of light produced by an electron incidentalong the path 5, is shown by the dash-line 6.

The conventional method of applying the aluminum backing film 3 is asfollows. After the phosphor for the screen 2 has been settled on theinner face of tube 1, a thin film of organic material is laid down invery intimate contact with the phosphor screen. When dry, the tube isevacuated and a layer of aluminum metal about 0.1 thick is evaporatedonto the continuous organic film. Thus, the thin aluminum film iscontinuous and highly reflecting. It is almost opaque optically and is agood conductor.

Air is then admitted to the tube and it is backed to a temperature ofabout 350 C., when the organic film material is oxidised to gas andsubstantially all removed from the space between the screen 2 and thealuminum film 3. The aluminum film 3 is thus left in very intimatecontact with the screen 2 and should perform all the functions requiredof it, as enumerated above.

However, it is found in practice that it is difiicult to remove all theorganic material. A residue is left which either impairs the reflectionefficiency of the aluminum film or absorbs some of the energy of theincident electrons, or usually both. This reduces the efficiency of thescreen by a serious factor, of perhaps 40%.

This reduction in efliciency has been proved by comparing the efliciencyof a screen prepared with an aluminum backing film 3, by theconventional method described, with that of a screen for which thealuminum backing film 3 was prepared separately, kept very clean andhighly-reflecting and then floated over the phosphor screen 2 andsettled on it. Other things being kept constant, a screen prepared bythis latter method is some 50% more eflicient than a screen prepared bythe conventional method.

This latter method is not a practicable method, however, since althoughthis aluminum film prevents light leaving the inner surface of thescreen 2 and also acts as a good electrical conductor, it causesdispersion of light reflected by it.

The reason appears to be that the film 3 cannot be laid on the screen 2in sufficiently intimate contact with the screen surface and hence,because of a gap between the phosphor screen 2 and the reflectingaluminum film 3, the reflected light is able to spread laterally by anamount that is sufficient to impair the image definition.

It is not practicable to evaporate the aluminum film 3 directly onto thesurface of phosphor screen 2, because the granular nature of the screenprevents a coherent film from forming. The aluminum film so formed isconsequently not satisfactory for preventing light leaving the backsurface of the screen 2 or for maintaining uniform the electricalpotential of the screen.

Furthermore, the aluminum, if evaporated normal to the surface of thescreen 2, tends to penetrate between phosphor grains and reach lowerlevels of the screen 2 where, by absorbing light, it can do more harmthan good.

The reason for the deterioration of the fluorescent im age produced bythe screen 2, as viewed by the photographic emulsion layer 4, is alsoshown in FIG. 1.

If fluorescent light, produced by an electron incident along the path 5,were to continue in the same direction, it would travel along the pathand be absorbed by the emulsion layer 4 at, say, a point X. The distancetravelled by the light ray from the point of its generation to the pointof its use is indicated in the figure by the distance A. This distanceis largely determined by the thickness of the glass end-face 1. Thisthickness is naturally reduced to the minimum value consistent with themechanical strength of the tube envelope as a whole.

Any divergence of the light ray from the path 5', such as by the path 6shown, results in the light ray being utilised at the point X. Theresultant dispersion of the light image in the emulsion layer 4 isindicated by the distance B.

The improved method of applying an aluminum backing film to a phosphorscreen will now be exemplified with reference to FIG. 2, in which figurecorresponding parts to those of FIG. 1 are indicated by the samereference numerals.

The inner surface of the end-face 1 of the tube is prepared and thecrystalline phosphor screen 2 deposited thereon in known manner.

A layer of aluminum is next evaporated directly onto the inner surfaceof the phosphor screen 2, without an intermediate organic film, in sucha way that it reaches substantially only the upper surfaces of thesurface layer of crystals of the screen 2. This is done according to apreferred method, by evaporating from several separate sources, forexample, source A and source B, of aluminum from, in this example, fourdirections separated by 90 and directed at a small angle to the surface,in this case less than 30.

Thus, a large part of the upper surfaces of the crystals of screen 2 arecoated with a very clean efficient reflecting aluminum layer which willperform the light-reflecting function very efliciently. However, it isnot a continuous conducting layer, and it does not suppress all thebackwardly-directed light. This discontinuous, intimate aluminum layeron the upper surfaces of the phosphor crystals is shown by the brokenheavy line 7 in FIG. 2.

It is estimated that this aluminum reflecting layer 7 will reflect some80% of the reversely-directed fluorescent light from screen 2 back intothe required direction for viewing with, possibly, less unwanted lateralspread than when the conventional aluminium backing method is used.

To provide for the requirements mentioned of preventing light fromleaving the inner face of screen 2 and of providing a continuousconducting layer for main taining uniform the screen potential, afurther film 3' than when the conventional aluminum backing method isdeposited on top of the film 7, either by the conventional techniquedescribed or by the floating technique described above. This provides acontinuous conducting aluminum film which prevents light from leavingthe screen 2 in the reverse direction. Even if the film 3' is a poorreflector this fact will not greatly matter because the film 3' is onlyrequired to reflect a small proportion, say 20% of the totalbackwardly-directed light.

The floating-on technique described, which provides the continuousaluminum film 3' directly onto the discontinuous layer 7, is preferredto the conventional evaporating method, which would require first anintermediate organic layer to be provided on top of the layer 7, sincethe floating on method provides a very clean film free from residualorganic matter which would result from the evaporating method and whichwould absorb energy from the incident electrons.

The method according to the invention for applying the compositealuminum backing film 7, 3' is considered to have limited importanceonly as applied to cathode ray tubes, but to have great importance asapplied to electron image intensifying tubes, particularly tubesemploying several stages of image intensification in cascade in the onetube.

I claim:

1. A method of providing electron-permeable backing layers for acrystalline fluorescent layer disposed on a transparent carrier in anelectron discharge tube, said method comprising the steps of evaporatinga metallic material from a plurality of spaced sources disposed at asmall angle to the fluorescent layer surface to form a discontinuous,high light-reflecting, first backing layer solely on the surface of saidcrystals remote from the carrier, and subsequently disposing a secondcontinuous planar, metallic, backing layer over and in local contactonly with said first backing layer.

2. A method according to claim 1, in which each of said two backinglayers consists of aluminum.

3. A method according to claim 1, using four said spaced sourcesseparated by and directed to the fluorescent layer surface at an angleof less than 30.

4. A method according to claim 1, in which a thin layer of organicmaterial is disposed on said first backing layer, said second backinglayer is evaporated onto the surface of the organic material layer andthe organic material is subsequently removed by heating.

5. A method according to claim 1, in which said second backing layer isformed apart from said fluorescent layer and is floated over and settledonto the fluorescent layer and said first backing layer.

References Cited by the Examiner UNITED STATES PATENTS 2,960,416 11/1960Reed 313-92 X 3,058,842 10/1962 Kahan 117107 3,087,085 4/ 1963 Turner313-92 3,095,319 6/1963 Williams 117-107 3,141,106 7/1964 Kapany 313-92ALFRED L. LEAVITT, Primary Examiner.

MURMY KATZ, Examiner.

A. H. ROSENSTEIN, Assistant Examiner.

1. A METHOD OF PROVIDING ELECTRON-PERMEABLE BACKING LAYERS FOR ACRYSTALLINE FLUORESCENT LAYER DISPOSED ON A TRANSPARENT CARRIER IN ANELECTRON DISCHARGE TUBE, SAID METHOD COMPRISING THE STEPS OF EVAPORATINGA METALLIC MATERIAL FROM A PLURALITY OF SPACED SOURCES DISPOSED AT ASMALL ANGLE TO THE FLUORESCENT LAYER SURFACE TO FORM A DISCONTINUOUS,HIGH LIGHT-REFLECTING, FIRST BACKING LAYER